Photochromism is defined as the reversible photoinduced transformation of a chemical species between two isomers having different absorption spectra [1]. The photochemical reaction is accompanied by a difference in properties other than the absorption of light such as the emission of light, refractive index, polarization, redox potentials, dipole moments, host-guest interactions and chemical reactivity. The changes in these properties implies that photoresponsive compounds can contribute to the advancement of numerous, diverse applications, where it is desirable that a given property is regulated. Applications include, but are not limited to, photonic devices such as (1) actinometers, (2) sensors and dosiometers, (3) novelty items such as inks, paints and other dyes, (4) variable transmission filters, (5) optical information storage systems, (6) molecular switches that can be incorporated into molecule-based wires, circuitry and machinery, (7) opto-electronic systems, (8) reversible holographic systems, (9) electro-optical devices such as waveguides, (10) the light-induced delivery of biologically, medically and synthetically relevant compounds, and (11) photoregulation of polymers. [1] Organic Photochromic and Thermochromic Compounds; Crano, J. C., Guglielmetti, R. J., Eds.; Plenum Press: New York, 1999; Vols. 1 and 2. M. Irie, in Molecular Switches, (Ed.: B. L. Feringa), Wiley-VCH, Weinheim, Germany, 2001, pp. 37-62; Special issue on photochromism: M. Irie, Chem. Rev. 2000, 100, 1685-1716.
Electrochromic molecules which change color when electrochemically oxidized or reduced are also well known in the prior art [2]. For example, compounds exhibiting electrochomism, including “dual mode” compounds having both electrochromic and photochromic properties, are described in applicant's PCT application No. PCT/CA2003/001216 (WO 2004/015024) which is hereby incorporated by reference. [2] Electrochromism: Fundamentals and Applications, Monk, P. M. S.; Mortimer, R. J.; Rosseinky, D. R., Eds., VHC: New York, 1995.
Numerous optical technologies such as waveguiding, data storage, variable reflectance in eyewear and filters, and sensors rely on the non-linear optical (NLO) properties of materials [3]. There has been a recent and enormous growth in the interest in NLO materials and some estimates claim that over one third of the existing electronic technologies currently used for data transmission and processing will be replaced by the faster electro-optic and photonic analogues. The success of these devices requires the development of new functional NLO materials with large and rapid NLO responses. NLO properties originate from molecules that have strong charge transfer excitations within non-centrosymmetric structures due to a polarisable π-conjugated framework, where electron donor (‘D’) and acceptor (‘A’) groups at the ends of the linear π-pathway creates an asymmetric charge distribution. [3] Di Bella, S. Chem. Soc. Rev. 2001, 30, 355. Verbiest, T.; Houbrechts, S.; Kauranen, M.; Clays, K.; Persoons, A. J. Mater. Chem. 1997, 7, 2175.
In systems that undergo “gated” photochromism, irradiation with light does not trigger a molecular transformation unless another external stimulus such as electricity, other photons, heat, or a chemical is applied before or during the irradiation period. By combining more than one input stimulus in molecular switching technologies, “logic-based” devices can be developed. In reactivity-gated photochromism or electrochromism, an initial chemical reaction must occur to convert the compound from a non-photo- or electroactive state to a photo- or electroactive state. Such systems may be particularly useful for sensing and dosiometry applications.
Molecular architectures that incorporate the 1,3,5-hexatriene motif are often photoresponsive and undergo reversible ring-closing and ring-opening reactions. Hexatriene compounds such as diarylethenes make up an important class of photoswitchable compounds [4] and many of the derivatives are also electroactive [5]. These particular compounds typically undergo thermally irreversible photoreactions with a high degree of fatigue resistance. They are the focus of numerous current research efforts. Previous reports of reactivity-gated photochromism using diarylethenes describe systems that operate based on the fact that the presence of the gate input affects the quantum yield of the ring-closing and ring-opening reactions by biasing the conformational equilibrium of the systems [6]. In many cases the effects are small. [4] M. Irie, in Molecular Switches, (Ed. B. L. Fering a), Wiley-VCH, Weinheim 2001, 37-60.[5] Peters, A.; Branda, N. R. Chem. Commun. 2003, 954. Gorodetsky, B.; Samachetty, H.; Donkers, R. L.; Workentin, M. S.; Branda, N. R. Angew. Chem. Int. Ed. 2004, 43, 2812. Koshido, T.; Kawai, T.; Yoshino, K. J. Phys. Chem. 1995, 99, 6110. Peters, A. Branda, N. R. J. Am. Chem. Soc. 2003, 125, 3404. Zhou, X.-H.; Zhang, F.-S.; Yuan, P.; Sun, F.; Pu, S.-Z.; Zhao, F.-Q.; Tung, C.-H. Chem. Lett. 2004, 33, 1006. Moriyama, Y.; Matsuda, K.; Tanifuji, N.; Irie, S.; Irie, M. Org. Lett. 2005, 7, 3315. Brown, W. R.; de Jong, J. J. D.; Kudernac, T.; Walko, M.; Lucas, L. N.; Uchida, K.; van Esch, J. H.; Fering a, B. L. Chem. Eur. J. 2005, 11, 6414. Brown, W. R.; de Jong, J. J. D.; Kudernac, T.; Walko, M.; Lucas, L. N.; Uchida, K.; van Esch, J. H.; Feringa, B. L. Chem. Eur. 12005, 11, 6430. Guirado, G.; Coudret, C.; Hliwa, M.; Launay, J.-P. J. Phys. Chem. B. 2005, 109, 17445. Tsujioka, T.; Kondo, H. App. Phys. Lett. 2004, 83. 937.[6] Takeshita, M.; Irie, M. J. Chem. Soc., Chem. Comm. 1996, 1807. Takeshita, M.; Soong, C. F.; Irie, M. Tetrahedron Lett. 1998, 39, 7717. Irie, M.; Miyatake, O.; Uchida, K.; Eriguchi, T. J. Am. Chem. Soc. 1994, 116, 9894.
The development of novel variations of the versatile 1,3,5-hexatriene architecture and convenient methods to prepare them is an important goal. The incorporation of a donor-it-acceptor motif (D-π-A) which can be reversibly created and broken in a controlled manner as part of the photochromic reaction of the novel architecture would be advantageous for the development of new functional NLO materials. It would also be beneficial to develop new compounds suitable for reactivity-gated photochromism or electrochromism, including gated systems enabling controlled release of small molecules and the like.